user=> (doc defmulti)
-------------------------
clojure.core/defmulti
([name docstring? attr-map? dispatch-fn & options])
Macro
Creates a new multimethod with the associated dispatch function.
The docstring and attribute-map are optional.
Options are key-value pairs and may be one of:
:default
The default dispatch value, defaults to :default
:hierarchy
The value used for hierarchical dispatch (e.g. ::square is-a ::shape)
Hierarchies are type-like relationships that do not depend upon type
inheritance. By default Clojure's multimethods dispatch off of a
global hierarchy map. However, a hierarchy relationship can be
created with the derive function used to augment the root ancestor
created with make-hierarchy.
Multimethods expect the value of the hierarchy option to be supplied as
a reference type e.g. a var (i.e. via the Var-quote dispatch macro #'
or the var special form).

Scott Lowe
added a comment - 10/May/12 5:16 PM New patch: '0001-CLJ-835-Refine-doc-string-for-defmulti-hierarchy-opt.patch' 10/May/12.
I've attached a new doc string patch in response to Andy Fingerhut's request posted here: https://groups.google.com/forum/?fromgroups#!topic/clojure-dev/i7H82fJYL-U
Andy's request stated "Attached patch seems to have spelling mistakes, and perhaps could be worded more clearly."
I hope my new patch is an improvement upon what was there.
This patch supersedes '0001-Clarify-docstring-for-defmulti.patch' from 14/Sep/11.

This is a million times better than what was there before, but (who knew!?) it could be better. A couple points of contention:

The order under the :hierarchy section seems off. I would start with an overview of what a hierarchy is including a definition of the global hierarchy map (it's mentioned, but glossed over). I'd then move on to an example impl. Finally, I'd then add the discussion starting "Multimethods expect..." immediately followed by an example.

In the isa? example it would be great to show the return value.

You mention that the hierarchy should be supplied as a reference, which is correct, however you talk about #'. The link between e.g. var and #' could be a bit more explicit. Maybe the sentence about #' could be parenthetical?

It's ok to finish with a very small example of its use in defmethod, in fact a simple example specifying methods for ::shape and ::circle would clarify the intent nicely.

Fogus
added a comment - 16/Aug/12 2:49 PM This is a million times better than what was there before, but (who knew!?) it could be better. A couple points of contention:

The order under the :hierarchy section seems off. I would start with an overview of what a hierarchy is including a definition of the global hierarchy map (it's mentioned, but glossed over). I'd then move on to an example impl. Finally, I'd then add the discussion starting "Multimethods expect..." immediately followed by an example.

In the isa? example it would be great to show the return value.

You mention that the hierarchy should be supplied as a reference, which is correct, however you talk about #'. The link between e.g. var and #' could be a bit more explicit. Maybe the sentence about #' could be parenthetical?

It's ok to finish with a very small example of its use in defmethod, in fact a simple example specifying methods for ::shape and ::circle would clarify the intent nicely.